A continuing program is being conducted for the routine analyses of feed samples for chlorinated hydrocarbons, polychlorinated biphenyls, organo-phosphates; lead, arsenic, cadmium, mercury, nitrate, and aflatoxins. The objective of this program is to document the concentrations of these potential contaminants in natural ingredient laboratory animal diets and to establish the maximal permissible concentrations that can be attained on a practical basis.